1. Field of the Invention
The present invention relates to data communications. In particular, the present invention relates to automatic detection of signal pair crossover on a high performance serial bus system.
2. The Prior Art
Background
The IEEE 1394-1995 standard has revolutionized the consumer electronics industry by providing a serial bus management system that featured high speeds and the ability to “hot” connect equipment to the bus; that is, the ability to connect equipment without first turning off the existing connected equipment. Since its adoption, the IEEE 1394-1995 standard has begun to see acceptance in the marketplace with many major electronics and computer manufacturers providing IEEE 1394-1995 connections on equipment that they sell.
However, as technologies improved, the need to update the IEEE 1394-1995 standard became apparent. A new standard is being proposed at the time of the filing of this application, herein referred to as the P1394b standard. Improvements such as higher speeds and longer connection paths will be provided. It is contemplated at the time of this filing that cable lengths of up to 100 meters may be possible using the P1394b standard. Furthermore, the connections between 1394 devices may be established using wiring previously installed in buildings compliant with appropriate regulatory codes.
In the discussion that follows, it will be necessary to distinguish between the various standards that are being proposed as of the date of this application. Additionally, it will be necessary to distinguish hardware and packet transmissions that are compatible with the P1394b standard and not earlier standards.
Thus, the term “Legacy” will be used herein to refer to the IEEE 1394-1995 standard and all supplements thereof prior to the P1394b standard. Thus, for example, a Legacy node refers to a node compatible with the IEEE 1394-1995 standard and all supplements thereof up to, but not including, the P1394b standard.
Furthermore, in the discussion that follows cable physical layers (PHY)s that are compatible with the P1394b standard may be referred to in various ways, depending upon the context the PHY is operating in and the capability of the PHY. For example, a PHY that has circuitry compatible with the P1394b standard but not any previous standards will be referred to as a B only PHY. Also, a PHY that is compatible with both the P1394b standard and all predecessors and is communicating with only devices compatible with the P1394b standard will be referred to as B PHYs. Finally, a PHY that is communicating with both Legacy devices and devices compatible with the P1394b standard will be referred to as a border device, border PHY, or border node. Finally, a communications systems that has only B PHYs attached will be referred to as a B bus.
P1394b Cabling
FIG. 1 shows a prior art diagram of a cable 100 according to the P1394b standard. Cable 100 includes a first signal pair 105 covered by a shield 106, a second signal pair 107 covered by a shield 108, and a power pair 104. The pairs are then enclosed in an outer shield 102, and extruded in an outer jacket 101.
According to the P1394b standard, the first and second signal pairs 105 and 107 form a differential pair, with different information being transmitted through each signal pair. As is appreciated by those of ordinary skill in the art, when a cable is connected to P1394b-compliant device, the device will receive information on a signal pair designated as twisted pair A (TPA), and will transmit on a signal pair designated as twisted pair B (TPB).
FIG. 2 shows a diagram of a prior art P1394b connection. FIG. 2 includes a PHY 1 having a connection point TPA and TPB, and a PHY 2 having a connection point TPA and TPB. Together, a TPA and TPB pair comprises a port. PHYs 1 and 2 are connected via cable 200, which has a first signal pair 202 and a second signal pair 204.
As can be seen by inspection of FIG. 2, because the PHYs transmit and receive on different signal pairs, the first and second signal pairs must be “crossed-over” to properly couple the PHYs. Thus, in FIG. 2, first signal pair 202 is connected to PHY 2's TPB and must be crossed-over to be connected to PHY 1's TPA. The same is true for second signal pair 204. As is appreciated by those of ordinary skill in the art, the signal pair crossover is typically accomplished within the P1394b cabling itself.
In P1394b, a PHY engages in simultaneous transmission and reception called ‘dual simplex’. This is different from Legacy, which transmits data on TPB and a “strobe” signal on TPA. Its peer port receives the data on TPA and the strobe on TPB. Legacy operates in “half duplex”, whereby previous arbitration determines the direction of the data flow on any given connection. The two connected ports then prime themselves so that one port transmits and one port receives as described above. Because of the strobe signal, Legacy operation requires the use of a crossover, and there is no possibility for operation if the crossover is not provided. To facilitate backwards compatibility, P1394b specifies that the port transmits on TPB and receives on TPA.
FIG. 3 is a diagram of a prior art P1394b system. Where similar designations are used herein, they are intended to refer to substantially similar matter. FIG. 3 shows what is referred to as a “cluster” of PHYs 1, 2, and 3. The PHYs may represent P1394b-compliant devices such as a computer, video camera, and a mass storage device. In FIG. 3, the PHYs are each connected to each other by a cable 200 in a point-to-point fashion to form the cluster. When devices are connected as in FIG. 3, the crossover normally provided in the cabling is sufficient to provide a proper connection for P1394b devices.
However, one of the advantages of the P1394b standard is the ability to run long lengths of cable (as far as 100 m) and connect P1394b devices throughout a house which has pre-existing wiring, for example. This leads to the problem illustrated in the diagram of FIG. 4.
FIG. 4 shows PHY 1 connected to PHY 2, through a wall connection. As is known by those of ordinary skill in the art, when a house is wired for point-to-point connections through a wall, typically wiring is used that will not implement a crossover. FIG. 4 shows such a straight-through wire 400 having conductors 402 and 404 which are not crossed-over internally, and external wall connections pairs 406 and 408.
As can be seen by inspection of FIG. 4, if a user attempts to connect PHY 1 to PHY 2 using a cable such as straight-through cable 400, the TPB1 will be coupled to TPB2 through the wall connection as shown with the solid black conductor, and TPA1 will be coupled to TPA2 as shown with the dashed conductor path. As is appreciated by those of ordinary skill in the art, such a connection will not function properly, and PHY 1 will not communicate properly with PHY 2.
To solve the problem of FIG. 4, two types of patch cords are used which are commonly available. One patch cord implements the crossover and one does not. This solution is tolerable in commercial buildings, where professional network managers ensure that the correct type of cable is used in each circumstance, and also where devices are typically connected semi-permanently.
However, this situation of FIG. 4 leads to frustrating difficulties in the consumer environment, where the subtleties of “cross-over” and “straight-through” patch cables are bewildering.
Furthermore, patch cords have certain drawbacks. For example, patch cords require much trial-and-error to locate and correct the connection that is not crossed-over, causing much user confusion and frustration. Furthermore, in modern homes and businesses which utilize P1394b-compliant devices, often all devices are coupled to a central router through in-wall wiring, making any troubleshooting effort even more difficult.
Hence, there is a need for a method and apparatus for automatically healing a crossover problem in a P1394b-compliant system. Furthermore, there is a need for a method and apparatus for automatically healing a crossover in the P1394b environment without the need for different types of patch cords.